Recent data indicate that a variety of vasodilators (such as acetylcholine) cause endothelium-dependent relaxation of vascular smooth muscle. This phenomenon has not been well studied in the cerebral circulation, however. The objectives of the proposed research, therefore, are to investigate two aspects of endothelium-smooth muscle interaction in blood vessels supplying the rat brain: Specific Aim 1: to test the hypothesis that acetylcholine causes endothelium-dependent relaxation of cerebral arterioles, and Specific Aim 2: to test the hypothesis that sustained contraction after rat carotid endothelial denudation is due to fundamental changes in smooth muscle contractile function. Studies of intracerebral arterioles (Specific Aim 1) will be carried out in vitro on isolated perfused penetrating vessels with diameters of 25-40 microns. It will be determined whether acetylcholine causes these vessels to dilate. Then the effect of endothelial removal on the acetylcholine response will be studied. Smooth muscle contractile function after endothelial removal (Specific Aim 2) will be studied in the versatile and accessible rat carotid balloon injury model. This balloon de-endothelialization injury produces a well-characterized proliferation of smooth muscle cells in the subintimal layer and sustained smooth muscle contraction. Smooth muscle contractile function will be assessed at intervals after endothelial denudation in ring segments generating isometric force in a myograph, and in isolated segments of perfused common carotid arteries. The health-related significance of the proposed research is: 1. basic data on cholinergic mechanisms in cerebral arterioles should provide crucial information about the recently described cholinergic vasodilator pathway arising from the nucleus basilis. This cholinergic pathway may be important in Alzheimer's Disease (7). 2. Since both rat carotid balloon injury and cerebral vasospasm are characterized by smooth muscle cell proliferation and sustained vasoconstriction, basic data derived from the proposed studies should further an understanding of delayed ischemic deficit after subarachnoid hemorrhage.